1. Field of the Invention
The present invention relates to a closure membrane for containers which comprises an outer film and inner film with a joining layer between these two films and a sealing seam zone.
2. Background Art
It is known to place substances, for example foodstuffs, in containers and to seal off beaker-type containers with a lid such as a closure membrane, or to seal off pouches by folding and sealing. Closure membranes may e.g. be made of papers, metal foils or plastic films, or laminates of several of the same or different metals, plastics and papers. The purpose of the closure membrane is to tightly seal the container in order to prevent ingredients of the contents escaping or preventing substances from outside entering the container. There are cases in which it is undesirable to have the container completely sealed by the closure membrane, this in order to allow the pressure in the interior of the container to adjust to that of the surrounding atmosphere. This concerns in particular cases in which the contents are placed in the container, the container is sealed, and the lidded container is subjected to a thermal treatment, such as, pasteurization, sterilization or ultrapasteurization. The intention is to prevent a positive pressure forming in the container that could cause the closure membrane to tear or rupture. This can be avoided by only partially sealing the closure membrane after filling the container, and allowing substances to escape at the non-sealed region between the edge of the container and the closure membrane. Subsequently, in another stage of the process, the closure membrane is then completely sealed onto the container.
In another version, described in EP-B 0 156 404, an opening for adjusting the pressure inside the container is provided on the container; after the thermal treatment, this opening is hermetically sealed by a polymer compound which melts on application of heat. This version, however, suffers the disadvantage that the polymer compound that can be melted is situated at an exposed area which is not protected from abrasion, and the opening for compensating for pressure differences provides a direct connection between the interior of the container and the surroundings.